Downgrade Java 1.7 to 1.6.
Merge pull request #3 from hyunsik/DOWNGRADE_JVM

Downgrade Java 1.7 to 1.6.
Replace Objects and BitSet of JAVA 7 with substitutes.
Improve descriptions in site pages and add the configuration page.
Merge branch 'master' of github.com:tajo-project/tajo into NEW_OPTIMIZER
Fixed typo errors in some documents.
Merge branch 'master' of github.com:tajo-project/tajo into NEW_OPTIMIZER
Migrate Java 1.7 to 1.6.